Merge PR #503

Since the refactoring 7af66b0 the Cheroot worker threads are named
like `('CP Server Thread-<id>',)`. A leftover comma transformed the
string to a tuple.

This patch fixes this by removing the trailing comma when setting the
thread name as `str`.

Ref:
* #503
* #516

Co-Authored-By: Carey Metcalfe <carey@cmetcalfe.ca>
Co-Authored-By: Christoph Heer <christoph@thelabmill.de>

cheroot/workers/threadpool.py

@@ -178,7 +178,7 @@ def start(self):
         for worker in self._threads:
             worker.name = (
                 'CP Server {worker_name!s}'.
-                format(worker_name=worker.name),
+                format(worker_name=worker.name)
             )
             worker.start()
         for worker in self._threads:
@@ -228,7 +228,7 @@ def _spawn_worker(self):
         worker = WorkerThread(self.server)
         worker.name = (
             'CP Server {worker_name!s}'.
-            format(worker_name=worker.name),
+            format(worker_name=worker.name)
         )
         worker.start()
         return worker